When polio was a household word and iron lungs were the stuff of nightmares, parents rushed their children out to get shots as soon as the Salk vaccine became available.

Today, when the danger seems past, many children in Maryland, including Montgomery and Prince George's Counties, are not receiving immunizations polio or the other dangerous, preventable childhood diseases - measles, rubella, mumps, diphtheria, whooping cough and tetanus.

Health officials say a new generation of parents who do not remember the epidemics of other years is becoming increasingly lax about having their children immunized, despite free immunization programs throughout the state.

"Vaccination levels have fallen so low that there is a very real risk of epidemics," said Dr. John J. Witte, director of the Immunization Division of the Center for Disease Control, Department of Health, Education and Welfare. "People assume that the diseases have been conquered and that there is nothing further to worry about. This complacency is based on a dangerous misconception. Unvaccinated children are still at risk. Where there are large numbers of unvaccinated children, epidemics can strike suddenly and spread quickly."

The number of children who have been immunized in Maryland has reached "an intolerably low statewide level of 58 per cent of 2-year-olds completely protected" against preventable childhood diseases, according to a recent preliminary study by the Maryland Childhood Immunization Action Committee. While there is no actual "safe" level of protection, health officials said the danger of epidemics persists until 80 per cent of the nation's young children have been immunized, and even then isolated outbreaks are possible.

In Montgomery County, the preliminary study showed, only 64.5 per cent of two-year-olds were fully protected and in Prince George's County, 60.2 per cent of the children in the same age group had been adequately immunized against childhood diseases.

In both counties, children were most often lacking in immunizations against polio, the study showed.

In Montgomery County, two-year-olds were most often immunized against measles (96 per cent); followed by rubella (93.5 per cent); mumps (87.9 per cent); and diphtheria, whooping cough and tetanus (73 per cent). Only 64.5 per cent of 2-year-old children had been immunized against polio, according to the preliminary survey.

Similarly, Prince George's County two-year-olds were most often immunized against measles (90.8 per cent); followed by rubella (89.2 per cent); mumps (81.2 per cent); and diphtheria, whooping cough and tetanus (70.4 per cent).The percentage of children immunized against polio was the lowest at 60.2 per cent.

But even though more children are getting immunized against measles than any other dangerous preventable childhood disease, the number of the children getting measles is on the rise in both Montgomery and Prince George's Counties.

There have been 42 reported cases of measles in Montgomery County this year so far, compared with four reported cases for all of 1976 and no reported cases for 1975, according to the Montgomery County Health Department. In Prince George's County, there is a similar increase in case of measles. There were 33 reported cases so far this year, compared with 17 in 1976 and 13 in 1975, health department officials there said.

The next highest incidence of preventable childhood diseases in both counties was for mumps. Five cases have been reported in Prince George's County so far this year. Last year, Prince George's County had seven reported cases of mumps, and in 1975, 17 cases of mumps were reported to the health department, officials said.

In Montgomery County, there have been no reported cases of mumps so far this year, according to the health department, but three cases were reported last year and there were eight reported cases in 1975.

Rubella (German meales) was not reported so far this year or last year in either county. It is a very contagious disease which can have serious consequences when contracted by women in the early stages of pregnancy. Many of their offspring have been found at birth to be suffering from one or more birth defects such as congenital cataract, heart disease, deaf-mutism or mental deficiency. The last reported case occurred in Montgomery County in 1975.

There were also no reported cases of diphtheria or tetanus in either county in the last three years. Only one case of pertussis has been reported in recent years - in Prince George's County in 1975. That disease, which struck a reported 4,624 people in Maryland and killed 260 people in 1923, has all but disappeared. "We attribute the stand off to vaccines and vaccines only," McAvinue said. "And now that the barriers are being jeopardized by the apathy of parents," he warned, there is "a real risk of re-runs of past performances."

Of all preventable childhood diseases, paralytic polio has been dormant the longest, even though statistics show that is the disease children are least protected against.

The last case of paralytic polio in either county occurred in Montgomery County when a 4-month-old baby contracted the disease in 1973, according to the state health department. Until that case, paralytic polio had not hit suburban Maryland for more than a decade. But in the 1950's 172 people in Montgomery County were stricken.

Prince George's County has not had a single reported case of paralytic polio since 1960, when three people contracted the disease. But in the 50's 162 people were stricken.

Then why, if fewer children are being immunized against polio than any other preventable childhood disease, have there been no outbreaks in Montgomery and Prince George's Counties for so long? Some health officials call it luck.

According to Dr. Myron Levine, director of the Center for Vaccine Development at the University of Maryland Hospital in Baltimore, "We've been very lucky. There probably is not wild polio virus in the United States. . . (but) when it comes, we do have problems."

The wild polio virus, (as differentiated from the live "weakened" virus used in the oral Sabin vaccine), is very much alive in most of Central America and the Caribbean, Levine said. "Most of the less developed countries have polio all the time," he said. "The virus is constantly being introduced by illegal transportation across the border." A couple of outbreaks that "came over the boarder from Mexico" have been reported in Texas over the past 10 years, Levine said, but "we almost eradicated it from the United States, with the exception of these few."

But even with introduction of the virus from outside the United States, the odds are against being stricken with paralystic polio, Levine sais. "Only about 1 per cent or 1/2 per cent of those infected would have paralytic disease. Most people would have silent (non-paralytic) infection," which could go unnoticed forever, he said.

Another reason polio may be dormant is "a certain amount of herd immunity," has been built up by successive generations, Levine said.